1. Technical Field
This invention relates to a method and apparatus for positioning blocking elements in a collimator which is located in a radiation emitting device, and more particularly to a method and an apparatus for positioning blocking elements in a collimator used for radiation treatment.
2. Description of the Related Art
Radiation-emitting devices are generally known and used, for instance, as radiation therapy devices for the treatment of patients. A radiation therapy device generally comprises a gantry which can be swiveled around a horizontal axis of rotation during the course of a therapeutic treatment. A linear accelerator is located in the gantry for generating a high-energy radiation beam (typically, of electrons or photons, that is, X-rays) for therapy. During treatment, this radiation beam is trained on a zone of a patient lying in the isocenter of the gantry rotation.
Contour collimators can be used in radiation therapy. Malignant tissue is destroyed by exposing it to a powerful radiation beam, and at the same time, it is important that healthy tissue not be damaged. For this purpose, in radiation therapy equipment, a contour collimator is placed between the source of radiation and the irradiated area. This limits the radiation exposure to essentially the malignant tissue.
Usually, contour collimators comprise movable aperture plates which are driven by electric motors. In recent years, multileaf collimators have contained two stacks of aperture plates. Each aperture plate is driven by a separate electrical motor such that apertures of various shapes are created between the opposing aperture plates. Such a multileaf contour collimator has been described in U.S. Pat. No. 5,165,106. This contour collimator has rotatably mounted aperture plates and the plates of each stack can be rotated around one axis to define an opening. Similarly, linear contour collimators can comprise two stacks of aperture plates which are linearly movable against each other in the same plane. These plates are also driven by electric motors.
Before treatment, an object (e.g., a patient) may be scanned with a computer tomograph (CT) and/or treatment may be simulated with a diagnostic X-ray unit. These devices localize the spot in the body to be irradiated and identify surrounding critical organs. The physician determines a method of treatment based on patient weight, patient age, and type and size of the diseased area. Data from the CT and the simulator along with the machine type are used in the treatment planning process to calculate dose levels to the treatment site and radiation exposure to surrounding tissue. When the physician approves a plan, it is transferred to a verify-and-record system. The contour collimator is then adjusted according to the treatment plan. Correct calibration of the contour collimator is critical.
Calibration of the contour collimator can be done by moving the aperture plates to defined positions and then setting a controller to values corresponding to these defined positions. This is done, for instance, by temporarily replacing the radiation beam by a light beam, so that a projection of the opening can be seen on graph paper in the plane of the object. Values read from the graph paper are then input in the controller as reference points. This method, while providing adequate calibration, is very time consuming. In light of this, there is a need for a simple and easy way to calibrate a collimator and more particularly to calibrate the positions of the plurality of aperture plates in a multileaf contour collimator.